Hermetic cap layers formed on low-k films by plasma enhanced chemical vapor deposition

ABSTRACT

A method of forming a cap layer over a dielectric layer on a substrate including forming a plasma from a process gas including oxygen and tetraethoxysilane, and depositing the cap layer on the dielectric layer, where the cap layer comprises a thickness of about 600 Å or less, and a compressive stress of about 200 MPa or more. Also, a method of forming a cap layer over a dielectric layer on a substrate including forming a process gas by flowing together about 200 mgm to about 8000 mgm of tetraethoxysilane, about 2000 to about 20000 sccm of oxygen (O 2 ), and about 2000 sccm to about 20000 sccm of carrier gas, generating a plasma from the process gas, where one or more RF generators supply about 50 watts to about 100 watts of low frequency RF power to the plasma, and about 100 watts to about 600 watts of high frequency RF power to the plasma, and depositing the cap layer on the dielectric layer.

BACKGROUND OF THE INVENTION

Semiconductor device geometries have dramatically decreased in sizesince these devices were first introduced several decades ago.Paralleling this development, semiconductor device clock speeds, oftenmeasured in terms of frequency, have gone from kilohertz (kHz) tomegahertz (MHz) to gigahertz (GHz), requiring electronic signals totravel across device interconnects with increasing speed. As devicegeometries shrink, and device speeds increase, the need to reduceincreased power consumption and signal slowdown due to the RC time delayof the interconnects becomes increasingly important.

Two significant components of the RC time delay in interconnects are theresistance (R) of the conductive material (e.g., a metal such as Al orCu) used in the interconnect, and the capacitance (C) of the dielectricmaterials that insulate the interconnect from other conductive regions.Progress has been made on reducing the resistance of the interconnectby, for example, switching from less conductive aluminum to moreconductive copper. Progress has also been made on the development ofdielectric materials having a lower dielectric capacity (i.e., low-κmaterials) to reduce the capacitance side of the RC time delay.

A number of low-κ dielectric materials, and techniques for integratingthem into semiconductor devices, have been developed. These include, forexample, incorporating fluorine or other halogens (e.g., chlorine,bromine) into a silicon oxide layer. Other low-κ materials includespin-on-dielectrics such as hydrogen silsesquioxane (HSQ), andcarbon-silicon containing dielectrics that are deposited by chemicalvapor deposition (e.g., plasma CVD), to form silicon-oxygen-carbon(Si—O—C) dielectric films. These materials are often deposited at lowtemperature (e.g., about 100° C. to about 200° C.) and low density, andoften have substantially high porosity.

The high porosity of many of these low-κ dielectric films makes themsusceptible to being infiltrated by contaminants in an ambientatmosphere. For example, water vapor (i.e., moisture) can quicklypermeate a porous dielectric material and increase the dielectricconstant of the layer. In some instances, the increase in κ-value causedby the moisture can make the dielectric layer higher κ thanconventional, undoped oxide layers. Thus, there is a need for methods ofprotecting low-κ dielectric layers from moisture infiltration thatincreases the dielectric constant of the layers.

BRIEF SUMMARY OF THE INVENTION

One embodiment of the invention includes a method of forming a cap layerover a dielectric layer on a substrate. The method includes forming aplasma from a process gas that includes oxygen and a silicon containingprecursor. The method also includes depositing the cap layer on thedielectric layer, where the cap layer has a thickness of about 600 Å orless, and a compressive stress of about 200 MPa or more.

Another embodiment of the invention includes a method of forming a caplayer over a dielectric layer on a substrate. This method includesforming a process gas by flowing together about 200 mgm to about 8000mgm of a silicon containing precursor, about 2000 to about 20000 sccm ofoxygen (O₂), and about 2000 sccm to about 20000 sccm of carrier gas. Themethod also includes generating a plasma from the process gas, where oneor more RF generators supply about 50 watts to about 100 watts of lowfrequency RF power to the plasma, and about 100 watts to about 600 wattsof high frequency RF power to the plasma. The method further includesdepositing the cap layer on the dielectric layer, where the cap layerhas a compressive stress of 200 MPa or more.

Another embodiment of the invention includes a system for forming a caplayer over a dielectric layer on a substrate. The system includes ahousing configured to form a processing chamber. The system alsoincludes a gas distribution system to flow about 200 mgm to about 8000mgm of a silicon containing precursor, about 2000 to about 20000 sccm ofoxygen (O₂), and about 2000 sccm to about 20000 sccm of carrier gasthrough a gas distribution faceplate and into the processing chamber.The system further includes a plasma generation system configured toform a plasma within the processing chamber, where the plasma generationsystem comprises one or more RF generators that supply about 50 watts toabout 100 watts of low frequency RF power to the plasma, and about 100watts to about 600 watts of high frequency RF power to the plasma. Inaddition, the system includes a substrate holder configured to hold thesubstrate about 350 to about 450 mils from the gas distributionfaceplate within the processing chamber, where the cap layer formed hasa thickness of about 600 Å or less.

Additional features are set forth in part in the description thatfollows, and in part will become apparent to those skilled in the artupon examination of the following specification or may be learned by thepractice of the invention. The features and advantages of the inventionmay be realized and attained by means of the instrumentalities,combinations, and methods particularly pointed out in the appendedclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of device layers including a cap layerformed according to embodiments of the invention;

FIGS. 2A-C are cross sectional views of device layers including a trenchfor a dual-damascene interconnect and a cap layer formed according toembodiments of the invention;

FIG. 3 is a flowchart illustrating methods of forming a cap layeraccording to embodiments of the invention;

FIGS. 4A and 4B are cross-sectional views of an embodiment of a chemicalvapor deposition apparatus that may be used in conjunction withembodiments of the invention;

FIG. 5 is a plot of humidity induced stress versus cap layer thicknessfor cap layers formed with varying compressive stresses on a 200 mmwafer; and

FIG. 6 is a plot of humidity induced stress versus cap layer thicknessfor cap layers formed with varying compressive stresses on a 300 mmwafer.

DETAILED DESCRIPTION OF THE INVENTION

The present invention includes methods of forming a cap layer on anunderlying dielectric layer to prevent moisture from infiltrating theunderlying layer and increasing its κ-value (among other adverseeffects). The cap layer may also act as a barrier for gaseouscontaminants (e.g., NH_(x)) diffusing through a porous dielectric layerand poisoning photoresist layers. These and other diffusion problems arebecoming increasingly significant as development trends for low-κdielectric layers often favor the increasing porosity of these layers.

Cap layers made according to embodiments of the methods of the inventionare formed with compressive stresses (e.g., about 200 MPa to about 300MPa or more) that make the cap layer an effective diffusion barrier toprevent moisture and other gaseous compounds from moving into and out ofunderlying dielectric layers. Experiments show that cap layers formedaccording to the invention with these compressive stresses can act aseffective moisture barriers at thicknesses of about 200 Å or less.Because these cap layers can serve as effective moisture/contaminantbarriers while remaining thin, their impact on the κ value of thedielectric layer is reduced. Some exemplary device structures thatinclude the cap layers formed according to embodiments of methods of theinvention are now described.

Exemplary Device Structures

FIG. 1 shows a cross-sectional view of device layers including a caplayer formed according to embodiments of the invention. The layersinclude an underlying layer 102, which may be, for example, asemiconductor substrate (e.g., a Si substrate), a metal interconnectlayer (e.g., a Al or Cu layer), or a silicide layer (e.g., an alloy ofsilicon and Ti, Ni or Co in contact with an underlying source, drain orgate electrode), among other kinds of layers. Underlying layer 102 maybe the lowest, base layer of the device (e.g., a 200 mm or 300 mmsubstrate wafer) or it may lie on top of other layers.

Above the underlying layer 102 is dielectric layer 104. Dielectric layer104 is formed from a low-κ dielectric material having a dielectricconstant (κ) of about 4 or less (e.g., about 3.5 or less, about 3 orless, about 2.5 or less, about 2 or less, etc.). These materials mayinclude, for example, silicon-oxygen-carbon materials (e.g., siliconoxycarbide (SiOCH)), parylene, amorphous fluorinated carbon materials,and hydrogen silsesquioxane (HSQ), among other low-κ dielectricmaterials. The materials may be deposited on underlying layer 102 usinga variety of techniques including thermal CVD, plasma CVD, and spin-ontechniques, among others.

As noted above, many of the materials and deposition techniques used toform dielectric layer 104 create a layer having high porosity and lowdensity. The density of dielectric layer 104, as measured by acomparison of the layer's etch rate with a thermal oxide layer (i.e.,the wet etch rate ratio (WERR)) may be about 1.5 or more (e.g., about 2or more, about 3 or more, etc.). The low-density of dielectric layer 104allows gases such as water vapor and nitrogen to diffuse quickly throughthe layer.

In order to decrease the diffusion of gases through dielectric layer104, a cap layer 108 is formed on top of the dielectric layer 104. Caplayer 108 may be made from silicon dioxide that is deposited by plasmaCVD from a mixture of a silicon precursor (e.g., silane (SiH₄),tetraethylorthosilane (TEOS), octamethylcyclotetrasiloxane (OMCTS),etc.), oxygen (e.g., O₂), and a carrier gas (e.g., helium (He)).Additional details about the method of forming the cap layer aredescribed below.

Cap layer 108 may be substantially thinner than the underlyingdielectric layer 104. For example, embodiments of the cap layer 108 mayhave a thickness of about 600 Å or less, or 200 Å or less. Embodimentsalso include a thickness for the cap layer from about 200 Å to about 600Å.

Cap layer 108 may be formed with compressive stress, which is believedto enhance the ability of cap layer 108 to block water vapor and othergases from entering and exiting the dielectric layer 104. The cap layer108 may have compressive stress of 200 megapascals (MPa) or more (e.g.,about 200 MPa to about 300 MPa, about 300 MPa or more, etc.).

Referring now to FIGS. 2A-C, cross sectional views of device layersincluding a trench 216 for a dual-damascene interconnect and a cap layer208 formed according to embodiments of the invention are shown. Thetrench 216 may be formed according to a dual-damascene processes thatstarts with the formation of barrier layer 203 (e.g., a silicon nitridelayer), first dielectric layer 204, etch-stop layer 205, and seconddielectric layer 206 on underlying layer 202 as shown in FIG. 2A. Afirst photoresist 210 may be formed on top of the second dielectriclayer 206 and developed to form a mask pattern that exposes a portion ofthe underlying dielectric layer 206. Dielectric layer 206 may then beetched down to etch-stop layer 205 to form first trench 212.

Following the formation of first trench 212, the mask layer formed byfirst photoresist 210 may be removed and cap layer 208 may be formedover the dielectric layer 206 as shown in FIG. 2B. A second photoresist214 (e.g., a DUV photoresist) may be formed over the cap layer 208 anddeveloped to form a mask pattern that exposes a portion of the cap layer208 inside trench 212 as shown in FIG. 2C.

The cap layer 208 may act as a barrier that prevents gaseouscontaminants from diffusing up from the etch-stop layer 205 and throughthe sides of dielectric layer 206 into the second photoresist 214. Someof these contaminants (e.g., NH_(x)) may react (sometimes called “resistpoisoning”) with materials in the second photoresist 214 to create“footings” (i.e., undeveloped portions of photoresist formed along thewalls and bottom of trench 212 that interfere with the patterning offeatures underneath the photoresist.)

In addition, an anti-reflective coating (not shown) may be formed on thecap layer 208 before second photoresist layer 214 is deposited. The ARCcoating reduces the amount of light reflecting from underlying layersinto photoresist layer 214 and reducing the sharpness of thesubsequently formed mask pattern. Cap layer 208 prevents materials inthe ARC coating from penetrating into the underlying, porous dielectriclayer 206.

After the mask pattern is formed in second photoresist layer 214, asecond trench 216 may be etched into the first dielectric layer 204 downto the barrier layer 203. The combination of the first trench 212 andsecond trench 216 forms the gap which may be filled with a conductivematerial (not shown) to form a dual-damascene interconnect.

Exemplary Method of Forming a Cap Layer

FIG. 3 shows a flowchart illustrating methods of forming a cap layeraccording to embodiments of the invention. The method starts byproviding a substrate 301 upon which the cap layer will be formed. Thesubstrate may be, for example, a conventional 200 mm or 300 mm siliconwafer. Additional layers, such as the low-κ dielectric layer shown inFIG. 1, may be formed on the substrate from a previous process (notshown). The dielectric layers may include gaps (e.g., shallow trenchstructures, deep trench structures, etc.) such as those shown anddescribed above in FIGS. 2A-C.

An optional silicon pretreatment step may be useful for sealing channelsand pores in the upper portions of porous dielectric layers prior to theformation of the cap layer. Reactive species from oxygen containinggases (e.g., O₂) can infiltrate pores and channels of porous low-κdielectric layers during the deposition of the cap layer and damage thedielectric material. Filling these pores with silicon containingmaterials may help prevent degradation of the dielectric layer duringthe formation of the cap layer.

The pretreatment step prior to the formation of the cap layer includesflowing a silicon gas mixture 302 that may include, for example, asilicon containing precursor (e.g., TEOS) and carrier gas (e.g., He).The silicon gas mixture is used to form a plasma 303 whose reactivespecies (e.g., Si) may form deposits in the pores and on the surface ofthe dielectric layer.

Following the pretreatment step (if performed) a process gas may flow304, which supplies precursor materials for the cap layer. The processgas may include a silicon containing percursor (e.g., silane (SiH₄),tetraethylorthosilane (TEOS), octamethylcyclotetrasiloxane (OMCTS),etc.), an oxygen containing gas (e.g., oxygen (O₂)) and a carrier gas(e.g., helium). In one embodiment, a combination of TEOS flowing at arate of about 200 to about 8000 milligrams per minute (mgm), O₂ flowingat a rate of about 2000 to about 20,000 sccm, and helium flowing at arate of about 2000 to 20,000 sccm make up the process gas being fed tothe process chamber.

A plasma may be formed from the process gas 306 through a plasmageneration system coupled to or integrated with the process chamber. Theplasma generation system may include one or more RF generators that, fora 200 mm wafer, may generate about 40 to about 60 watts (e.g., about 50watts) of low frequency (e.g., about 350 to about 550 kHz) RF power tothe plasma, and about 100 to about 300 watts of high frequency (e.g.,about 10 to 15 MHz) RF power to the plasma. When 300 mm wafer substratesare used, the power generated may be increased to about 80 to about 120watts for the low frequency RF power, and about 200 to about 600 wattsfor the high frequency RF power.

Reactive species in the plasma may then be deposited over a dielectriclayer on the substrate to form the cap layer 308. The cap layerdeposition rate may range from about 800 to about 1000 Å/min (e.g.,about 875 Å/min) to deposit a cap layer having a thickness of about 600Å or less (e.g., about 200 to about 600 Å, about 200 Å or less, etc.).

Exemplary Process Chamber

An exemplary CVD process chamber in which embodiments of the method ofthe present invention can be carried out is shown in FIGS. 1A and 1B.FIG. 1A shows a vertical, cross-sectional view of a CVD system 10,having a vacuum or processing chamber 15 that includes a chamber wall 15a and chamber lid assembly 15 b.

CVD system 10 contains a gas distribution manifold 11 for dispersingprocess gases to a substrate (not shown) that rests on a heated pedestal12 centered within the process chamber. During processing, the substrate(e.g. a semiconductor wafer) is positioned on a flat (or slightlyconvex) surface 12 a of pedestal 12. The pedestal can be movedcontrollably between a lower loading/off-loading position (depicted inFIG. 1A) and an upper processing position (indicated by dashed line 14in FIG. 1A and shown in FIG. 1B), which is closely adjacent to manifold11. A centerboard (not shown) includes sensors for providing informationon the position of the wafers.

Deposition and carrier gases are introduced into chamber 15 throughperforated holes of a conventional flat, circular gas distribution orfaceplate 13 a. More specifically, deposition process gases flow intothe chamber through the inlet manifold 11 (indicated by arrow 40 in FIG.1B), through a conventional perforated blocker plate 42 and then throughholes 13 b in gas distribution faceplate 13 a.

Before reaching the manifold, deposition and carrier gases are inputfrom gas sources 7 through gas supply lines 8 (FIG. 1B) into a mixingsystem 9 where they are combined and then sent to manifold 11.Generally, the supply line for each process gas includes (i) severalsafety shut-off valves (not shown) that can be used to automatically ormanually shut-off the flow of process gas into the chamber, and (ii)mass flow controllers (also not shown) that measure the flow of gasthrough the supply line. When toxic gases are used in the process, theseveral safety shut-off valves are positioned on each gas supply line inconventional configurations.

The deposition process performed in CVD system 10 may be aplasma-enhanced process. In a plasma-enhanced process, an RF powersupply 44 applies electrical power between the gas distributionfaceplate 13 a and the pedestal so as to excite the process gas mixtureto form a plasma within the cylindrical region between the faceplate 13a and the pedestal. (This region will be referred to herein as the“reaction region”). Constituents of the plasma react to deposit adesired film on the surface of the semiconductor wafer supported onpedestal 12. RF power supply 44 is a mixed frequency RF power supplythat typically supplies power at a high RF frequency (RF1) of 13.56 MHzand at a low RF frequency (RF2) of 450 KHz to enhance the decompositionof reactive species introduced into the vacuum chamber 15. In a thermalprocess, RF power supply 44 would not be utilized, and the process gasmixture thermally reacts to deposit the desired films on the surface ofthe semiconductor wafer supported on pedestal 12, which is resistivelyheated to provide thermal energy for the reaction.

CVD system 10 may also be used for thermal deposition processes. Duringa thermal deposition process, a heat transfer liquid is circulatedthrough the walls 15 a of the process chamber to maintain the chamber ata constant temperature to prevent condensation of liquid precursors andreduce gas phase reactions that could create particles. A portion ofthese heat-exchanging passages in the lid of chamber 15 is shown in FIG.1B. The passages in the remainder of chamber walls 15 a are not shown.Fluids used to heat the chamber walls 15 a include the typical fluidtypes, i.e., water-based ethylene glycol or oil-based thermal transferfluids. This heating (referred to as heating by the “heat exchanger”)beneficially reduces or eliminates condensation of undesirable reactantproducts and improves the elimination of volatile products of theprocess gases and other contaminants that might contaminate the processif they were to condense on the walls of cool vacuum passages andmigrate back into the processing chamber during periods of no gas flow.

The remainder of the gas mixture that is not deposited in a layer,including reaction byproducts, is evacuated from the chamber by a vacuumpump (not shown). Specifically, the gases are exhausted through anannular, slot-shaped orifice 16 surrounding the reaction region and intoan annular exhaust plenum 17. The annular slot 16 and the plenum 17 aredefined by the gap between the top of the chamber's cylindricalside-wall 15 a (including the upper dielectric lining 19 on the wall)and the bottom of the circular chamber lid 20. The 360.degree. circularsymmetry and uniformity of the slot orifice 16 and the plenum 17 helpachieve a uniform flow of process gases over the wafer so as to deposita uniform film on the wafer.

From the exhaust plenum 17, the gases flow underneath a lateralextension portion 21 of the exhaust plenum 17, past a viewing port (notshown), through a downward-extending gas passage 23, past a vacuumshut-off valve 24 (whose body is integrated with the lower chamber wall15 a), and into the exhaust outlet 25 that connects to the externalvacuum pump (not shown) through a foreline (also not shown).

The wafer support platter of the pedestal 12 (preferably aluminum,ceramic, or a combination thereof) is resistively-heated using anembedded single-loop embedded heater element configured to make two fullturns in the form of parallel concentric circles. An outer portion ofthe heater element runs adjacent to a perimeter of the support platter,while an inner portion runs on the path of a concentric circle having asmaller radius. The wiring to the heater element passes through the stemof the pedestal 12. Typically, any or all of the chamber lining, gasinlet manifold faceplate, and various other reactor hardware are madeout of material such as aluminum, anodized aluminum, or ceramic. Anexample of such a CVD apparatus is described in U.S. Pat. No. 5,558,717entitled “CVD Processing Chamber,” issued to Zhao et al. The U.S. Pat.No. 5,558,717 is assigned to Applied Materials, Inc., the assignee ofthe present invention, and is hereby incorporated by reference in itsentirety.

A lift mechanism and motor 32 (FIG. 1A) raises and lowers the heaterpedestal assembly 12 and its wafer lift pins 12 b as wafers aretransferred into and out of the body of the chamber by a robot blade(not shown) through an insertion/removal opening 26 in the side of thechamber 15. The motor 32 raises and lowers pedestal 12 between aprocessing position 14 and a lower, wafer-loading position. The motor,valves or flow controllers connected to the supply lines 8, gas deliverysystem, throttle valve, RF power supply 44, and chamber, substrateheating system and heat exchangers H1, H2 are all controlled by a systemcontroller 34 (FIG. 1B) over control lines 36, of which only some areshown. Controller 34 relies on feedback from optical sensors todetermine the position of movable mechanical assemblies such as thethrottle valve and susceptor which are moved by appropriate motors underthe control of controller 34.

In some embodiments, the system controller includes a hard disk drive(memory 38), a floppy disk drive and a processor 37. The processorcontains a single-board computer (SBC), analog and digital input/outputboards, interface boards and stepper motor controller boards. Variousparts of CVD system 10 conform to the Versa Modular European (VME)standard which defines board, card cage, and connector dimensions andtypes. The VME standard also defines the bus structure as having a16-bit data bus and a 24-bit address bus.

System controller 34 controls all of the activities of the CVD machine.The system controller executes system control software, which is acomputer program stored in a computer-readable medium such as a memory38. Preferably, memory 38 is a hard disk drive, but memory 38 may alsobe other kinds of memory. The computer program includes sets ofinstructions that dictate the timing, mixture of gases, chamberpressure, chamber temperature, RF power levels, susceptor position, andother parameters of a particular process. Other computer programs storedon other memory devices including, for example, a floppy disk or otheranother appropriate drive, may also be used to operate controller 34.

Experimental Results and Measurements

Hermeticity tests were conducted on cap layers deposited over low-κdielectric layers formed on 200 mm and 300 mm substrate wafers. Thesetests can gauge the effectiveness of cap layers in preventing moisturefrom reaching underling dielectric layers. The tests measure changes instress levels of a dielectric layer caused by the absorption/uptake ofmoisture by the layer. The change in the stress of the dielectric layeris found by measuring the change in the curvature of the substrate wafercaused by the change in stress in the dielectric layer. The change incurvature may in turn be measured by a change in the deflection path ofa laser beam that is being reflected off the surface of the wafer.

A quantitative relationship between the change in stress in thedielectric layer caused by the absorption/uptake of moisture and themeasured deflection of the laser beam may be given by:

${Stress} = {\frac{{ED}^{2}}{6\left( {1 - v} \right)t}\left( \frac{1}{R} \right)}$where: E=Young's modulus of the substrate

-   -   v=Poisson's ratio of the substrate    -   D=thickness of the substrate    -   t=thickness of the dielectric layer, and

${R = {2L\frac{\mathbb{d}x}{\mathbb{d}z}}},$

where: L=path length of the laser beam, and

$\frac{\mathbb{d}x}{\mathbb{d}z} = {{the}\mspace{14mu}{change}\mspace{14mu}{of}\mspace{14mu}{the}\mspace{14mu}{laser}\mspace{14mu}{beam}\mspace{14mu}{direction}\mspace{14mu}{along}\mspace{14mu}{the}\mspace{14mu} x\text{-}{axis}\mspace{14mu}{as}\mspace{14mu} a\mspace{14mu}{function}\mspace{14mu}{of}\mspace{14mu}{change}\mspace{14mu}{in}\mspace{14mu}{the}\mspace{14mu}{direction}\mspace{14mu}{of}\mspace{14mu}{the}\mspace{14mu}{orthogonal}\mspace{14mu} z\text{-}{axis}}$

The cap layers tested in the hermeticity tests were formed on low-κdielectric layers supported on 200 mm or 300 mm substrate wafers. Thelow-κ dielectric layers were formed from tensile, carbon-containingoxide materials that exhibit significant stress changes when exposed tomoisture (e.g., ambient moisture from room temperature air).

Cap layers of various thicknesses and compressive stress levels areformed on the dielectric layers for the hermeticity tests. Cap layershaving thickness of 200 Å, 400 Å, and 600 Å are each formed withcompressive stresses of −100 MPa, −200 MPa and −300 MPa on both 200 mmand 300 mm substrates, allowing hermeticity test to be performed on 18different wafer configurations. The hermeticity of each wafer was testedusing an “85/85 test,” where the wafers were stored at a temperature of85° C. in an atmosphere having 85% relative humidity for 17 hours priorto measuring the change in the stress level of the capped dielectriclayer.

The cap layers were deposited on 200 mm wafer substrates using a plasmaCVD chamber. The substrate and dielectric layer was heated to about 400°C. while a process gas that included TEOS flowing at about 300 mgm,oxygen (O₂) flowing at about 3000 sccm and helium flowing about 3000sccm, were used in the formation of the plasma. Plasma was energized bya combination high and low frequency RF power, where the low frequencypower was kept constant at about 50 watts while the high frequency powerwas adjusted between about 120 and 210 watts (i.e., 120 W, 160 W, or 210W) depending on the target compressive stress level (i.e., −100 MPa,−200 MPa, or −300 MPa) for the cap layer. For each cap layer deposition,the wafer substrate was spaced about 420 mils from the showerhead wherethe processes gases entered the deposition chamber. Table 1 lists someproperties of the cap layers formed on the 200 mm substrate wafers thatwere tested with the 85/85 test.

TABLE 1 Properties of Cap Layers Formed on 200 mm Wafers Stress Level(MPa)* Cap Layer Property −100 −200 −300 High Frequency RF 120 160 210Power (Watts) Deposition Rate 980 925 875 (Å/min) Uniformity, % 1 s 1.91.7 1.2 Uniformity, % R/2 3.0 2.8 2.6 RI 1.460 1.462 1.465 WERR @ 10 min4.2 2.9 2.5 100:1 HF Density (g/cm³) 2.221 2.228 2.23 *Negative MPavalues denote compressive stress while positive MPa values denotetensile stress

FIG. 5 plots the measured changes in stress level of the dielectriclayer on a 200 mm wafer following the 85/85 hermeticity test as afunction of thickness and compressive stress of the overlying cap layer.The plot shows that the cap layers formed with a compressive stress ofabout −300 MPa provided the highest levels of hermeticity (i.e., leastchange in the stress levels of the dielectric layer) at all three caplayer thicknesses (i.e., 200 Å, 400 Å, and 600 Å).

FIG. 6 plots the measured changes in stress level of the dielectriclayer on a 300 mm wafer following the 85/85 hermeticity test as afunction of thickness and compressive stress of the overlying cap layer.Similar to the results for the 200 mm wafer, cap layers formed withcompressive stress of −300 MPa showed the highest levels of hermeticityat all three cap layer thicknesses.

Having described several embodiments, it will be recognized by those ofskill in the art that various modifications, alternative constructions,and equivalents may be used without departing from the spirit of theinvention. Additionally, a number of well known processes and elementshave not been described in order to avoid unnecessarily obscuring thepresent invention. Accordingly, the above description should not betaken as limiting the scope of the invention.

Also, the words “comprise,” “comprising,” “include,” “including,” and“includes” when used in this specification and in the following claimsare intended to specify the presence of stated features, integers,components, or steps, but they do not preclude the presence or additionof one or more other features, integers, components, steps, or groups.

1. A method of forming a cap layer over a dielectric layer on asubstrate, the method comprising: forming a first plasma from a firstprecursor comprising silicon; pretreating a deposition surface of thedielectric layer with the first plasma, wherein the dielectric layer isa low-κ dielectric layer comprising silicon and carbon, and wherein thefirst plasma deposits silicon in at least one pore on the depositionsurface; forming a second plasma from a second process gas comprisingoxygen and a silicon containing precursor; and depositing the cap layeron the dielectric layer, wherein the cap layer comprises a thickness ofabout 600 Å or less, and a compressive stress of about 200 MPa or more.2. The method of claim 1, wherein the thickness of the cap layer isabout 200 Å to about 600 Å.
 3. The method of claim 1, wherein thethickness of the cap layer is about 200 Å or less.
 4. The method ofclaim 1, wherein the compressive stress of the cap layer is about 200MPa to about 300 MPa.
 5. The method of claim 1, wherein the compressivestress of the cap layer is about 300 MPa or more.
 6. The method of claim1, wherein the substrate comprises a 200 mm wafer.
 7. The method ofclaim 6, wherein the forming of the second plasma comprises: strikingthe second plasma using one or more RF generators, wherein the RFgenerators supply a low frequency RF power of about 50 watts, and supplya high frequency RF power from about 100 watts to about 300 watts. 8.The method of claim 7, wherein the low frequency power is about 350 toabout 550 kHz, and the high frequency power is about 10 to about 15 MHz.9. The method of claim 1, wherein the substrate comprises a 300 mmwafer.
 10. The method of claim 9, wherein the forming of the secondplasma comprises: striking the second plasma using one or more RFgenerators, wherein the RF generators supply a low frequency RF power ofabout 100 watts, and supply a high frequency RF power from about 200watts to about 600 watts.
 11. The method of clam 1, wherein the caplayer is formed at a rate of about 875 Å/min or less.
 12. The method ofclam 1, wherein the silicon containing precursor comprises silane,tetraethoxysilane, or octamethylcyclotetrasiloxane.
 13. The method ofclaim 1, wherein the method further comprises depositing a bottomantireflective coating (BARC) layer on the cap layer, wherein the caplayer prevents materials from the BARC layer from contaminating thedielectric layer.
 14. A method of forming a cap layer over a dielectriclayer on a substrate, the method comprising: pretreating a depositionsurface of the dielectric layer with the first plasma, wherein thedielectric layer is a low-κ dielectric layer comprising silicon andcarbon, and wherein the first plasma deposits silicon in at least onepore on the deposition surface; forming a process gas by flowingtogether about 200 mgm to about 8000 mgm of a silicon containingprecursor, about 2000 to about 20000 sccm of oxygen (O₂), and about 2000sccm to about 20000 sccm of carrier gas; generating a second plasma fromthe process gas, wherein one or more RF generators supply about 50 wattsto about 100 watts of low frequency RF power to the plasma, and about100 watts to about 600 watts of high frequency RF power to the plasma;and depositing the cap layer on the dielectric layer, wherein the caplayer has a compressive stress of 200 MPa or more.
 15. The method ofclaim 14, wherein the compressive stress of the cap layer is about 300MPa or more.
 16. The method of claim 14, wherein the cap layer has athickness of about 600 Å or less.
 17. The method of claim 14, whereinthe cap layer has a density of 2.22 g/cm³ or more.
 18. The method ofclaim 14, wherein the cap layer comprises undoped silicate glass. 19.The method of claim 14, wherein the dielectric layer is asilicon-oxygen-carbon (Si—O—C) layer.
 20. The method of claim 14,wherein the dielectric layer has a wet etch rate ratio (WERR) of 2.5:1or more.
 21. The method of claim 14, wherein the carrier gas is helium.22. The method of claim 14, wherein the silicon containing precursorcomprises silane, tetraethoxysilane, or octamethylcyclotetrasiloxane.23. A method of forming a cap layer over a dielectric layer on asubstrate, the method comprising: forming a first plasma from a firstprecursor comprising silicon; pretreating a deposition surface of thedielectric layer with the first plasma, wherein the first plasmadeposits a silicon in at least one pore on the deposition surface, andwherein the silicon is thinner than the dielectric layer; forming asecond plasma from a second process gas comprising oxygen and a siliconcontaining precursor; and depositing the cap layer on the dielectriclayer, wherein the cap layer comprises a thickness of about 600 Å orless, and a compressive stress of about 200 MPa or more.
 24. The methodof claim 23, wherein the dielectric layer is a low-κ dielectric layercomprising silicon and carbon.
 25. The method of claim 24, wherein thedielectric layer is a silicon-oxygen-carbon (Si—O—C) layer.
 26. Themethod of claim 1, wherein the dielectric layer is asilicon-oxygen-carbon (Si—O—C) layer.